Conventionally known is an electrophotographic printer which performs printing by modulating laser light in correspondence with an image signal then forming an electrostatic latent image with the modulated laser light and transferring a toner image onto a print sheet. A controller to control the operation of the printer is provided with a CPU such as microcomputer and a ROM holding a control program for the CPU and various data. As the ROM, a mask ROM is used for the sake of advantage in the cost of production.
The mask ROM is a low price device and appropriate for mass production. However, in a case where the control program stored in the mask ROM is somewhat changed due to, for example a change of specification of the apparatus, it is necessary to discard all the mask ROM and develop a mask ROM holding a new control program. This wastefulness is caused by the change of specification and it requires much time for development of new apparatus. For such reason, the conventional art cannot support diverse requirements from the market in a flexible manner.